1. Field of the Invention
The present invention generally relates to a circuit for interrupted feedback loop operation.
2. Problem to be Solved
Feedback loops are commonly used in electronic or electromechanical systems to regulate some parameter of interest. For example, a feedback loop can be used to regulate lamp current or power in a ballast for a fluorescent lamp. In many instances, it is necessary to interrupt the operation of a feedback loop. For example, the feedback loop used to regulate the lamp current or power in the ballast is interrupted in order to dim the lamp by modulating the time that the lamp is on and off. Pulse-width-modulation ("PWM") is typically used to achieve such modulation. Thus, when the lamp is on, the feedback loop is regulating lamp current or power in the ballast, but when the lamp is off, the feedback loop operation is interrupted.
One problem resulting from interruption of feedback loop operation is that the state variables in the feedback loop often decay during the interruption time and deviate from their steady state operating points. This decay is typically due to the RC (resistor capacitor) networks that normally function as filters in the feedback loop. When loop operation resumes, there is a transient associated with returning to steady state operation. This transient is generally worsened by the deviated state variables. For example, in one particular commercially available lighting system with dimming capability, the state variable is the lamp power and the decay occurs during PWM dimming. As a result of the decay, the switching frequency is initially relatively low because the feedback loop operates as though the lamp power is too low. This low switching frequency can saturate the resonant inductor thereby leading to excessive currents in the circuit and increased likelihood of visible flicker in the display.
One attempt at solving this problem is to clamp the state variable, during circuit interruption, to a value near its operating value. When operation resumes, the state variables are nearly correct and the transient is reduced in amplitude and/or duration. However, variations in circuit component values and variations in operating conditions can lead to different, and non-ideal, clamping values. As the operating conditions (such as input voltage) vary, the operating point of the state variable changes. Therefore, the fixed clamp value is only perfect for a single operating point and less than ideal for all other conditions.
It is therefore an object of the present invention to provide a feedback loop interruption circuit that solves the problem inherent in the prior art circuit discussed above.
Other objects and advantages of the present invention will be apparent to one of ordinary skill in the art in light of the ensuing description of the present invention.